Bug 21087: Do not store the password in ->store for existing patrons

When ->store is called on an existing patron we must not save the
password, but keep the one in DB instead.
There is a dedicated method (update_password) to call when the password
need to be updated

Signed-off-by: John Doe <you@example.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>